The invention relates to a simple press section in a paper or board machine.
In this application, by a separate press nip is meant a press nip in which press members lying against each other form only one press nip. A separate press nip can be a roll nip or an extended nip. A contrast to a separate press nip is a press provided with a centre roll, in which press the centre roll forms a press nip with at least two other rolls.
The function of a press section is to remove water from a web and make the web symmetric, with the result that the absorption and roughness properties of both sides of the web are symmetric. Moreover, the press section should be simple and efficient enough in order that it might be applied to several paper grades. Furthermore, the construction and maintenance costs of the press section must not become too high.
At high speeds in particular, the run of the web in the press section should be as straight as possible. The aqueous and weak web arriving at the press section does not withstand the dynamic forces caused by high speeds and changes of direction, but there occur web breaks and other operational malfunctions which cause shutdowns.
In Metso Paper, Inc.'s WO laid-open publication 99/60202, FIG. 1 shows a press section provided with two separate extended nips. The extended nips are formed by an upper shoe roll and a lower press suction roll. Both nips are double-felted, in which connection dewatering takes place in both nips in two directions. The web travels in the first nip between a first upper press felt, which is a pick-up felt, and a first lower press felt, and in the second nip between a second upper press felt and a second lower press felt. Thus, four press felts are used in this arrangement.
In Metso Paper, Inc.'s U.S. Pat. No. 5,522,959, FIG. 1 shows a press section provided with a pre-press nip and a separate extended nip. A pick-up felt runs through the pre-press nip and the extended nip and the web is transferred on support of the pick-up felt through both nips. The pre-press nip is formed between a pick up roll serving as an upper roll and a hollow-faced counter roll serving as a lower roll, and the extended nip is formed between a shoe roll serving as an upper roll and a smooth-faced counter roll serving as a lower roll. Thus, the web runs in the first nip between the pick-up felt and a press felt, in which connection dewatering of the web takes place in both directions. In the second nip, the web in turn runs between the smooth-faced counter roll and the pick-up felt, in which connection dewatering takes place mainly in one direction. The web is transferred from the smooth-faced counter roll of the second nip as a short open draw onto a drying wire. Two press felts are used in this arrangement.
In U.S. Pat. No. 4,988,410 (Voith), FIGS. 1 and 2 show a press section provided with two separate extended nips. The web is picked up from a forming section by a pick-up roll and passed between a pick-up felt and a first press felt into the first extended nip. Either the pick-up felt or the first press felt also runs through the second extended nip. The first extended nip is thus double-felted, in which connection dewatering of the web takes place in two directions. The second extended nip is in turn single-felted, in which connection dewatering of the web takes place mainly in one direction. The web is transferred from the surface of a counter roll of the second extended nip as a short open draw to a dryer section. The separation of the web from one press felt and its adherence to the opposite press felt between the first and second extended nips takes place here by means of a separate suction box. Two press felts are used in this arrangement.